For gathering precise surface-related information about an object, particularly topographical information, the surface of such object typically may be scanned using a laser beam which is moved over the object in a predefined manner and/or may be imaged using a camera unit in combination with a stereoscopic approach. Those methods for instance are provided by a geodetic measuring device like a terrestrial laser scanner or a total station, e.g. Leica P20 or Leica Multi Station 50, or by specified camera units. By scanning the object and by applying the stereoscopic method a so called (3D-) point cloud or an overall image may be created representing the object.
Such point cloud typically is derived by determining a distance for each measuring point and a correlated direction of the laser beam when determining the distance (laser scanner). The point-to-point resolution of such measuring points and the resulting point cloud, respectively, is defined by the speed of moving the laser beam on the surface and a triggering-interval for triggering single measurements (e.g. one for each measuring point).
Alternatively, parts of the point cloud are calculated from at least two images the stereo basis and the relative pose of which are known. Of course, the point cloud could be calculated based on a series of images. As a result of such stereoscopic calculations, the derived point cloud represents the surface of the object with corresponding point-to-point resolution.
In case of using a geodetic measuring device, as from one station point usually only a part of an object is measurable while other surface points are hidden, it becomes necessary to set up the measuring devices at least at two different positions with respect to the object such that in combination the whole surface of the object is measurable.
The surveying instrument needs direct line-of-sight to the object points to measure. In case of an obstruction, e.g. a tree in front of a building which occludes a part of the façade leads to a so called “scanning shadow”. In practice, in such a case the surveying instrument also is set up at a different position where direct line-of-sight to the missing parts is given. Therefore, more than one setup of the surveying instruments is needed and each additional setup takes time and reduces the productivity of the user.
Moreover, a full-dome-scan, i.e. a scanning area from 0° to 360° in horizontal and −45° to 90° in vertical direction, with a terrestrial laser scanner in highest resolution may take up to several hours. In this resolution the distance between the points in 100 meters is 1.0 mm. For every new setup of the instrument a full 360° panorama image is usually obtained which also takes several minutes. Thus, relocating a laser scanner or a similar surveying instrument (e.g. total station) and recording a second set of measuring data (second point cloud) is very time consuming and needs an expert at least for referencing the first point cloud relative to the second point cloud.
In case of measuring an object with a portable image capturing unit, data acquisition may be provided in a more flexible and faster manner. However, there still would remain regions at the object not being accessible with view to gathering corresponding adequate image data, e.g. high above the surface or of terrain which is difficult to access. Moreover, in order to reference the image data in a global coordinate system and to provide precise object information, particularly each capturing position would have to be assigned to a correspondingly captured image.
A further approach for gathering object data is based on a combination of scanner data with image data.
EP 1 903 303 B1 discloses a method of combining point cloud data with image data in order to fill up missing parts of the point cloud. The camera unit is used for recording a set of images which are split into a set of stereoscopic image pairs. Every image pair is processed independently. Moreover, the panorama image obtained by a laser scanner (the so-called “main image”) is used for pair wise matching with one stereoscopic image pair and thus providing adding dimensional information of the respective stereoscopic image pair to the point cloud. The whole process is performed in a post-processing step having all data of the set of images and the laser scanner ready for processing.
The main disadvantages of the methods above are on the one hand the huge amount of time being consumed with scanning the object and—on the other hand—remaining areas of the object which cannot be accessed in suitable manner, i.e. caused by obstacles in the line of sight of the scanner, or gathering images with recommended properties which cover all regions of interest, e.g. the roofs of buildings. Moreover, with respect to combining scanner data with image data, due to the post-processing and due to the independent processing of the stereoscopic images an error concerning the accuracy of point positions increases with the number of images not being directly related to the scanning point cloud.
In addition, above methods require quite intense manual settings and implementations (e.g. moving the camera or post-processing of data) and thus are prone to errors, require highly educated operators and consume relative much time.
As to a further aspect regarding determination of topographic data of terrain, a UAV is known to be used for that. Flight plans for such UAVs are mostly generated by selecting a terrain-area of interest, e.g. from a map, and define a particular altitude. A horizontal rectangle at the specified altitude is defined and provides for the borders for a resulting flight path. The flight path may be defined such that it covers the desired area of interest, wherein the acquisition of image data is set for providing a lateral overlap of the captured images regarding the respectively covered regions. The points of image acquisition are thus defined accordingly (depending on a required longitudinal overlap).
The precision of position information derived from such aerial images correlates with the ground sampling distance (GSD) (=distance on the ground which corresponds to e.g. one pixel in a captured image) and strongly depends on the altitude. The more distance is in between the UAV and the surface of the terrain, the larger the ground sampling distance, and the less precise the position data is. However, by reducing that distance also the danger of colliding with any kind of obstacle, e.g. trees, buildings and/or hills, rises accordingly. Moreover, such method of capturing images typically relates to capturing images from above and thus does not satisfactorily provide for gathering image data from vertical parts of the object.